Coking-resistant shaft/bushing mechanism for an exhaust gas recirculation valve

ABSTRACT

An exhaust gas recirculation valve assembly including a bushing and pintle shaft sub-assembly having a pintle shaft supported by a sleeve bushing, the contact area between the shaft and bushing being reduced such that coking deposits that form on the remaining contact area can be broken by the force of the valve actuator, thereby preventing sticking of the valve from coking accumulations. Either the shaft surface or the bushing surface is relieved in a pattern of incuse areas and residual bearing areas. A currently preferred pattern is a helix.

RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS

[0001] The present application claims priority from U.S. ProvisionalPatent Application, Serial No. 60/415,584, filed Oct. 2, 2002.

TECHNICAL FIELD

[0002] The present invention relates to pintle valves; moreparticularly, to exhaust gas recirculation (EGR) pintle valves forinternal combustion engines; and most particularly, to an arrangement ofthe pintle shaft and the pintle bushing to prevent valve failure fromshaft coking by exhaust gas components.

BACKGROUND OF THE INVENTION

[0003] Pintle valves are well known for use in controlling flow offluids, and especially gases. Some applications can expose a valve'sinternal surfaces and moving parts to fouling materials which can giverise to deposits, resulting in impaired action or outright failure ofthe valve. In a particularly severe application, exhaust gasrecirculation pintle valves on internal combustion engines are known toaccumulate coking deposits on the pintle shaft in the region of thepintle support bushing. Having this area of the pintle unobstructed bycoking deposits is a requirement for smooth, reliable action of thevalve in controlling exhaust gas flow. In severe cases, coking is knownto immobilize a pintle shaft in a bushing bore, resulting in completefailure of the valve.

[0004] It is known in the art to provide means for causing the pintleshaft to be scraped clean inherently by the axial motion of the shaft asthe valve is exercised. Such means typically are disposed upstream ofthe sensitive area, such as a shaft bushing or a valve actuator.

[0005] U.S. Pat. No. 5,511,531, issued Apr. 30, 1996, discloses anannular scraper element 74 for the purpose of scraping off any residuethat may accumulate on shaft 60 so that such residue is prevented frompassing onto actuator 12. At its center, scraper element 74 has acircular hole with which shaft 60 has a close sliding fit. The radiallyouter margin of element 74 is captured such that it is constrainedagainst any significant axial motion. A problem with such a scraper isthat it represents essentially a line contact around the shaft, and anymaterial that escapes past that line is then free to migrate or bedeposited further along the shaft and/or within the actuator. Further,scraper element 74 has no radial resilience and is subject to wear withuse, such that its scraping action becomes progressively less effectiveover time.

[0006] U.S. Pat. No. 4,253,642, issued Mar. 3, 1981, discloses anarrangement of wire bristles impinging radially on a valve pintle shaft“to clean threads or the like of the valve stem and remove debristherefrom.” A problem with using wire bristles is that, while the shaftmay be scrubbed thereby, the bristle pattern is entirely porous,allowing particulate-laden gases to permeate through and cause cokingdownstream of the bristles.

[0007] U.S. Pat. No. 6,212,881 B1, issued Apr. 10, 2001, discloses a“stopper” 72 having a central hole 72b through which the valve pintleshaft 56a passes. The diameter of the hole 72b is slightly larger thanthe diameter of shaft 56a. When the valve shaft is moved axially, theinner surface of the hole removes foreign matter such as deposits fromthe surface of the valve shaft. This is substantially the sameline-contact scraper mechanism as is disclosed in U.S. Pat. No.5,511,531 discussed above.

[0008] U.S. Pat. No. 5,666,932, issued Sep. 16, 1997, discloses aprogrammed feature of an engine start-up cycle wherein the EGR valve isviolently opened and then slammed closed, which purports to jar or shearany contaminant buildup so as to reduce such buildup. No contact meansis disclosed for actively scraping or otherwise mechanically removingcontaminant buildup.

[0009] U.S. Pat. No. 4,408,627, issued Oct. 11, 1983, discloses agate-type EGR valve operated by a linear actuator attached to a valveshaft. A pair of nylon scraper rings 28,29 are positioned around thevalve shaft “to remove any carbon deposits” from the valve shaft andprevent them from entering the actuator. The rings are separated by acompressed coil spring which urges the rings against respective axialsealing faces in the valve body.

[0010] U.S. Pat. No. 4,061,157, issued Dec. 6, 1977, discloses areciprocating valve having an outwardly facing conical surface disposedabout the valve stem, which surface terminates in a knife-like edge thatfunctions to remove accumulations of foreign material from exposedportions of the valve stem as it oscillates past the edge.

[0011] What is needed in the art is a means for limiting buildup ofcoking contaminants on a valve pintle shaft and/or shaft bushing toprevent immobilization of the shaft and failure of the valve.

[0012] It is a principal object of the present invention to increase thereliability of an exhaust gas recirculation valve by preventingunacceptable buildup of deposits on shaft and bearing surfaces thereof.

SUMMARY OF THE INVENTION

[0013] Briefly described, in an exhaust gas recirculation valve having apintle shaft supported by a sleeve bushing, the contact area between theshaft and bushing is minimized such that coking deposits that form onthe remaining contact area can be broken by the force of the valveactuator, thereby preventing sticking of the valve from cokingaccumulations. Either the shaft surface or the bushing surface isrelieved in a pattern of incuse areas and residual areas. A currentlypreferred pattern is a helix.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:

[0015]FIG. 1 is an elevational cross-sectional view of a prior artpintle-type valve;

[0016]FIG. 2 is a detailed view of Area “2” shown in FIG. 1, in partialcross-section, showing an improved pintle shaft in accordance with afirst embodiment of the invention;

[0017]FIG. 3 is a cross-sectional view of the embodiment shown in FIG.2, showing the pintle shaft in retracted position in the shaft bushing;

[0018]FIG. 4 is a cross-sectional view like that shown in FIG. 3,showing the pintle shaft in extended position in the bushing;

[0019]FIG. 5 is a cross-sectional view showing a second embodiment inaccordance with the invention; and

[0020]FIG. 6 is a view like that shown in FIG. 2, showing a thirdembodiment in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Referring to FIG. 1, a prior art pintle-type valve 10 includes avalve body 12 comprising a first chamber 14 and a second chamber 16separated by a first annular valve seat 18. A pintle shaft 24 having asurface 23 is slidably disposed in first axial bore 25 in bushing 26which is mounted in a second axial bore 28 in valve body 12. Valve head30 is fixedly attached to shaft 24 for axial movement therewith and ismatable with valve seat 18 to regulate flow across seat 18 betweenchambers 14 and 16 in response to actuation by solenoid actuatorassembly 32.

[0022] A pintle valve such as valve 10 may be mounted on an internalcombustion engine 34 for use as an exhaust gas recirculation valve inknown fashion. In such use, chambers 14 and 16 are fully exposed toengine exhaust gases. The purpose of the invention is to prevent gasesfrom chamber 16 from causing unacceptable levels of coking along shaftsurface 23 within bushing bore 25, which surface is subject toreciprocation into and out of bushing 26 during operation of the valve.

[0023] While not essential to the invention, the following explanationfor success of the invention is currently believed to be correct.

[0024] Coking products are chemical compounds similar to polymericcompounds in that they may be characterized in terms of shear strengthand modulus. Shear strength is a fundamental property of a compound andmay be used to determine a force required to break loose an assembly,such as a shaft in a bushing, which has seized due to formation ofcoking products at the shaft/bushing interface. The force required is afunction of the shear strength of the coking material and also the totalsurface area clogged by coking. Further, the axial force available froma solenoid actuator assembly, such as assembly 32, may be readilydetermined. Hence, immobilization of a shaft in a bushing by coking maybe prevented by limiting the area of shaft/bushing interface such thatthe shear resistance of the total coking products in that area canalways be overcome by the solenoid actuator.

[0025] Limiting the area cannot be done, however, simply by shorteningthe axial extent of the shaft in the bushing because the axial bearingextent is what guides the valve head into the seat and prevents wobbleand runout of the shaft in the bushing. Therefore, what is needed is ameans for maintaining or even extending the axial length of theinterface between the shaft and bushing while reducing verysubstantially the total interface area for debilitating accumulation ofcoking products.

[0026] Beneficially, it has been found that the shear strength of cokingproducts is very much a function of thickness and diminishesdramatically in inverse proportion to thickness, especially beyond avery thin “skin layer” having a thickness of about 0.05 mm. Any buildupbeyond the skin layer is crumbly and not very strong. Unfortunately, inprior art valves, the shaft clearance within the bushing bore is on theorder of about 0.05 mm, such that prior art valves are easily choked bycoking products.

[0027] Referring to FIGS. 2 and 3, a first embodiment of a bushing andpintle shaft sub-assembly 40 in accordance with the invention issuitable for substitution into a prior art valve assembly such asassembly 10. First embodiment 40 comprises a pintle bushing 26′. Animproved pintle shaft 24′ is disposed conventionally for axial actuationwithin bore 25′ in bushing 26′. A bushing cap 42 is urged againstbushing 26′ by a hookian element such as coil spring 44. A centralaperture 46 in cap 42 is close-fitting to surface 23′ of pintle shaft24′ to minimize escape of gases along bearing bore 25′.

[0028] Referring to FIG. 3, a first portion 48 of surface 23′ withinbore 25′ and below aperture 46 is relieved to define residual bearingareas 50 having a first radius 52 equal to the original radius 54 ofshaft 24′, and incuse areas 56 having a second radius 58 less than firstradius 52. A second portion 60 of surface 23′ within bore 25′ isunrelieved to provide for sealability of the shaft in the bushing, incooperation with cap 42. Preferably, the axial length 62 of portion 60is less than or equal to the shaft diameter (twice radius 52).

[0029] In a currently preferred embodiment, residual bearing area 50defines a clockwise helical pattern or “thread” having a relatively longpitch. Further, the helical width 64 of the residual area 50 ispreferably relatively narrow, to provide radial support for shaft 24′ inbore 25′ while simultaneously minimizing the surface of residual bearingarea 50 available for coking build-up. Further, the helical nature ofarea 50, being inclined to the path of axial travel of the pintle shaft,acts inherently to plow coking deposits from the surface of the bearingbore during actuation; and the helical pattern, being open at theinboard end of the bushing, permits scraped deposits to escape back intovalve chamber 16 from whence they are swept by the flow of gases throughthe valve.

[0030] Coking products should be expected to accumulate in incuse areas56; however, because the depth of such areas exceeds the thickness of askin layer, such crumbly and incompetent accumulations areinconsequential to causing the shaft to become immobilized in thebushing.

[0031] Referring to FIG. 4, shaft 24′ continues to be fully supported bybushing bore 25′ when the shaft is fully extended at valve closure, asshown in FIG. 1.

[0032] Referring to FIG. 5, in a second embodiment of a bushing andpintle shaft sub-assembly 40′ in accordance with the invention, incuseportions 56′ and residual bearing portions 50′ are formed in the bore25′ of bushing 26″ rather than in the pintle shaft 24 which may besmooth and unfeatured as in prior art pintle shaft 24.

[0033] In FIGS. 3-5, a cap spring 44 is assumed, but omitted forclarity.

[0034] Referring to FIGS. 2 and 6, a result of the helical arrangementshown in FIG. 2 is that a torque may be imposed on pintle shaft 24′during actuation. Such torque may be beneficial, causing the shaft torotate and thereby progressively clean the entire smooth bearing surfaceduring repeated actuations. In applications wherein no rotation isdesired, in a third embodiment 40″ as shown in FIG. 6, a secondopposite-handed helical residual bearing area 70 may also be providedsuch that there is no net torque on pintle shaft 24″ during actuation.Preferably, the helical crossings 72 are open to provide circulation ofgases and drainage of deposits along the incuse areas 56. An importantconsideration in providing a double-helix arrangement such as is shownin FIG. 6 is to continue to minimize the total residual area 50,70 madeavailable to coking, and to keep such total area small enough that theshear resistance of the accumulated coking deposits cannot exceed theaxial strength of the actuator.

[0035] While the invention has been described by reference to variousspecific embodiments, it should be understood that numerous changes maybe made within the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

What is claimed is:
 1. A coking-resistant bushing and pintle shaftsub-assembly for use in a pintle-type valve assembly, comprising: a) abushing having a bore therethrough; and b) a pintle shaft disposed insaid bore and having an outer surface, wherein at least one of said boreand said surface is at least partially relieved radially to define atleast one residual bearing area for radially supporting said shaft insaid bushing.
 2. A sub-assembly in accordance with claim 1 wherein saidpintle-type valve assembly includes a solenoid actuator assembly, andwherein the total area of said residual bearing area is selected suchthat coking deposits formed thereupon during use of said pintle-typevalve may be sheared by the actuating force of said solenoid actuator.3. A sub-assembly in accordance with claim 1 wherein a portion of saidresidual bearing area is formed in a helical shape.
 4. A sub-assembly inaccordance with claim 1 wherein a portion of said residual bearing areais formed in a shape comprising both a clockwise helix and acounterclockwise helix.
 5. A sub-assembly in accordance with claim 1wherein said valve is an exhaust gas recirculation valve for an internalcombustion engine.
 6. A sub-assembly in accordance with claim 1 whereinsaid radially relieved areas have a radial depth of at least 0.05 mm. 7.A coking-resistant pintle-type valve assembly, comprising: a) a valvebody including an internal chamber; b) a bushing disposed in a firstbore in said valve body and having a second bore extending between saidchamber and the exterior of said valve body; and c) a valve pintle shaftaxially disposed in said second bore and extending into said chamber andhaving a shaft surface, wherein at least one of said second bore andsaid shaft surface is at least partially relieved radially to define atleast one residual bearing area for radially supporting said shaft insaid bushing.
 8. An internal combustion engine, comprising acoking-resistant pintle-type exhaust gas recirculation valve assemblyhaving a valve body including an internal chamber, a bushing disposed ina first bore in said valve body and having a second bore extendingbetween said chamber and the exterior of said valve body, and a valvepintle shaft axially disposed in said second bore and extending intosaid chamber and having a shaft surface, wherein at least one of saidsecond bore and said shaft surface is at least partially relievedradially to define at least one residual bearing area for radiallysupporting said shaft in said bushing.